Methylamine Magnesium Borohydrides as Electrolytes for All-Solid- State Magnesium Batteries

Solid-state magnesium electrolytes may pave the way for novel types of rechargeable, sustainable, and cheap batteries with high volumetric and gravimetric capacities. There are, however, currently no solid-state magnesium electrolytes that fulfill the requirements for solid-state battery applications. Here, we present the synthesis, structure, and properties of six new methylamine magnesium borohydride compounds, a- and fi-Mg(BH4)2 center dot 6CH3NH2, Mg(BH4)2 center dot 3CH3NH2, and a-, a '- and fi-Mg(BH4)2 center dot CH3NH2. The fi-Mg(BH4)2 center dot CH3NH2 polymorph displays a record high Mg2+ ionic conductivity of cr(Mg2+) = 1.50 x 10-4 S cm-1 at room temperature. The high Mg2+ conductivity of fi-Mg(BH4)center dot CH3NH2 is facilitated by a one-dimensional chain-like structure interconnected by weak dihydrogen bonds and dispersion interactions, forming a migration pathway across the chains. The oxidative stability of Mg(BH4)2 center dot CH3NH2 is 1.2 V vs Mg/Mg2+, and the reversible plating and stripping were confirmed by cyclic voltammetry and symmetric cell cycling, revealing high stability toward magnesium electrodes for at least 50 cycles at 60 degrees C.

Automated summary

Solid-state magnesium electrolytes are crucial for the development of rechargeable batteries with high capacities, yet there is a lack of electrolytes that meet the requirements. In this study, six new compounds of methylamine magnesium borohydride were synthesized and their properties were investigated. One of the compounds showed a record high ionic conductivity of Mg2+ and exhibited stability towards magnesium electrodes.